Drill pipes for petroleum and natural gas drilling operation are manufactured in accordance with the standards published by American Petroleum Institute (API). According to Specification for Drill Pipe (API SPEC 5DP), there are only four grades of steel for drill pipes, namely E, X, G, S, corresponding to four levels of strength, i.e. 75 ksi, 95 ksi, 105 ksi and 135 ksi, respectively. To guarantee the impact performance of a drill pipe, the longitudinal full-size impact toughness of the drill pipe at room temperature shall be at least 54 J as stipulated by American Petroleum Institute in the Specification for Drill Pipe (API SPEC 5DP).
The operating environment for a drill pipe is getting increasingly harsher along with the development of the petroleum industry, such that an API standard drill pipe can no longer meet the progressively rigorous requirements of the drilling operation. In recent years, as deep and ultra-deep wells are developed, even higher requirements are imposed on the performances of a drill pipe. As such, the material of the drill pipe needs not only high strength, but also sufficient toughness reserve. Only in this way can it endure forced tension, forced torsion, impact vibration and the action of various alternate loads in overload operation, and be adapted to the requirements of using the dill pipe under a variety of special operating conditions. Hence, the standard of at least 54 J of longitudinal full-size impact toughness at room temperature specified for grade S drill pipes according to Specification for Drill Pipe (API SPEC 5DP) of American Petroleum Institute cannot satisfy the more and more rigorous requirements of the drilling operation any longer. Therefore, American Petroleum Institute proposes the performance requirements for grade PSL3 drill pipes in the standards: at least 100 J of longitudinal full-size impact toughness for grade S drill pipes at −20° C., i.e. the performance requirements of drill pipes having ultra-high toughness and high strength.
A Chinese patent application literature titled “High-strength Petroleum Drill Pipe and Manufacturing Process Thereof” (publication number: CN1690241A; publication date: Nov. 2, 2005) discloses a high-strength drill pipe having the following chemical composition in mass percentage: C: 0.20-0.30%; Si: 0.1-0.5%; Mn: 0.7-1.5%; Cr: 0.7-1.5%; Mo: 0.1-0.4%; V: 0.01-0.15%; and the balance of Fe and unavoidable impurities. A grade S drill pipe in conformity with Specification for Drill Pipe (API SPEC 5DP) of American Petroleum Institute may be made according to this patent application, wherein the impact strength of the pipe meets the requirement of at least 54 J of longitudinal full-size impact toughness at room temperature.